The present invention relates to a process for selecting solvents for forming films of ferroelectric polymers, the solvents selected, a composition for forming ferroelectric polymer films, and the films formed therewith, and in particular, ferroelectric polymer films suitable for use in a data processing device.
Ferroelectrics are a class of dielectric materials that can be given a permanent electric polarization by application of an external electric field. Use of ferroelectric materials in data processing devices is disclosed in U.S. Patent Application No. U.S. 2002/0044480 to Gudesen et al., which is directed to a ferroelectric data processing device comprising a thin film of ferroelectric material as a data-carrying medium. The film may be inorganic, a ceramic material, a polymer, or a liquid crystal. Gudesen does not, however, disclose how ferroelectric films are made. Use of ferroelectric polymers in data processing devices is also described, for example, by Y. Tajitsu et al., in “Investigation of Switching Characteristics of Vinylidene Fluoride/Trifluoroethylene Copolymers in Relation to Their Structures”, (Japanese Journal of Applied Physics, Volume 26, pp. 554–560, 1987).
Formation of ferroelectric films is disclosed in published U.S. Patent Application No. US 2002/0037399 and U.S. Patent Application No. 2002/0051879, both to Tamai et al, which are directed to forming ferroelectric films, including organic polymer films, that also contain microparticulates. These references state that the solvents used in forming a film is not critical (see paragraph [0054] and [0050], respectively).
In contrast, it is known that only certain vinylidene fluoride polymers are ferroelectric, and the presence of ferroelectricity is due at least in part to the history of the film, including the thermal history of the film and the solvent used to form the film. See, e.g., the Abstract of an article by Cho, in Polymer, Volume 15, p. 67 (1991). Recently, Tashiro et al., in Macromolecules, Volume 35, p. 714 (2002) performed a detailed structural analysis of the various vinylidene fluoride crystal morphologies. Vinylidene fluoride polymers occur in four distinct crystal morphologies, all monoclinic. Without intending to bound by theory, form I has essentially planar zigzag chains forming a polar structure in which CF2 dipoles are parallel to each other along the crystallographic b-axis. The chains are tightly packed and tend to form large crystals. In form II, the CF2 dipoles are packed in anti-parallel mode along the b-axis. Form II is therefore nonpolar and less tightly packed than form I. Form III is also a tightly packed polar unit cell, and is obtained by casting from highly polar (but not necessarily hydrogen bonding) solvents such as dimethylacetamide or dimethylformamide. Form III may also be obtained by annealing forms II or IV at high temperature. Finally, form IV is a polar structure in which the chains are packed in parallel mode. Form IV is also a desirable form from the standpoint of ferroelectric properties because it can interconvert with form II. Copolymers of vinylidene fluoride exhibit similar characteristics.
In addition to ferroelectricity, a number of other properties are important in the function and use of ferroelectric polymer films, including properties related to hysteresis (including saturation potential, coercive field strength, and permittivity); reliability (such as fatigue, aging, time dependence dielectric breakdown, imprint, and relaxation); and thermodynamic properties such as the Curie transition temperature of the film. Many factors can affect these properties, for example the composition of the polymers, and historical factors such as the heat load applied to a particular film, the process of by which a film is made, the solvent used to make a film, and mechanical stresses applied to the film, and the like. There accordingly remains a need in the art for methods for the manufacture of ferroelectric polymer films, in particular films suitable for use as memory devices, that are highly reproducible and that allow control of the properties of the film.